1. Field of Invention
The present invention relates to a lock-up device, particularly to a lock-up device for a torque converter for transmitting a torque and absorbing and attenuating a torsional vibration.
2. Background Information
Torque converters are often equipped with a lock-up device for directly transmitting a torque from a front cover to a turbine. The lock-up device includes a piston capable of being frictionally coupled to the front cover, a retaining plate fixed to the piston, a plurality of pairs of torsion springs supported by the retaining plate, and a driven plate configured to be elastically coupled to the piston through the plural torsion springs in a rotational direction. The driven plate is fixed to the turbine (e.g., Japan Laid-open Patent Application Publication No. 2008-138797).
The piston is herein mounted between the front cover and the turbine, and the torque of the front cover is configured to be transmitted to the piston when an annular friction facing fixed to the outer peripheral part of the piston is pressed onto a friction surface of the front cover. The torque transmitted to the piston is transmitted to the turbine through the torsion springs and the driven plate. Then, variation in torque to be inputted from an engine is absorbed and attenuated by the plural torsion springs composing part of a damper mechanism.
On the other hand, a lock-up device equipped with a damper mechanism having multistage torsional characteristics has been also already provided for effectively absorbing and attenuating variation in torque (e.g., Japan Laid-open Patent Application Publication No. 2011-179515).